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Failure of an I-Beam

Background: A 40’ long I-beam cracked longitudinally within the web region The cracked I-beam is one of the guide tracks of adrill stem top drive of a drill rig derrick structure. The I-beam is a W10x45 A572 Grade 50.

Figure 1: Longitudinal Crack in I-Beam

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Figure 2: Fatigue Striations and Dimples at the Center of Fracture Surface

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Summary: The failure mechanism of the cracked I-beam is low cycle high stress fatigue. Fatigue striations were clearly seen on the fracture surfaces together with dimples. Dimples indicate a ductile failure mechanism. The relatively thick fatigue striations, the presence of dimples on the fracture surface, and permanent bending of the web region where the crack formed, indicate high loading conditions. High dynamic bending stresses were generated by the normal contact force between the rollers of the direct drive system and track guide surface of the I-beam. Compressed and elongated grains on the microstructure at the track guide side edge also indicate a high normal contact force. Takeaway: High dynamic contact loads can result in low cycle fatigue failures.
Take Away: Temperature excursions coupled with improper cooling can lead to premature failures of TLEs. The microstructural changes in the material due to thermal exposure can further facilitate the loss of material under synergistic mechanisms of metal dusting and erosion.

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